The goal of this proposal is to understand the mechanistic aspects of cellular responses to L1 activity, to evaluate L1 -induced mutagenesis and its contribution to DNA damage and associated aging, to determine whether this contribution increases with age, and to find out potential variation of this contribution among individuals. These goals are important because the existing data indicate that the DNA damage in the form of double-strand breaks (DSBs) inflicted by the L1-encoded DNA endonuclease is potentially much greater than the insult to the genome caused by the element via insertional mutagenesis. We detected endogenous L1 expression in a number of somatic cells. Therefore this type of DNA damage has consequences for the whole organism, not only for its progeny. We also wish to address the L1 contribution to the DNA damage and associated aging in human stem cells. For these studies we will manipulate L1 expression levels and correlate L1 activity with the resulting DNA damage by western blot analysis, g-H2AX foci staining, and COMET assays in normal cell lines. We will use reporter assay to quantify the mutagenic potential of L1 elements. We will use cultured primary cell models to determine L1 potential to accelerate cellular senescence in somatic and stem cells. Reported ability of L1 elements to inflict DNA damage and its endogenous expression in a number of human tissues fundamentally changes our understanding of the significance of the L1 expression and its impact on the health of the host. This project is designed to determine whether ongoing low levels of L1 expression in somatic and stem cells lead to a steady accumulation of mutations due to the error-prone repair of the DSBs created by the L1 endonuclease. The direct health implication of this discovery would be L1 contribution to mammalian aging and/or age- associated diseases such as cancer. Due to the constant improvement of the health care there is a growing population of older people that is more susceptible to age-associated diseases. Understanding of the molecular mechanisms of aging would contribute to our potential to prevent the onset of aging or age- associated diseases.